This application relates to a transmission for driving an actuator for a vehicle transfer case wherein a unique stop defines ends of travel for the transmission.
Transfer cases allow a vehicle to be switched between two-wheel and four-wheel drive. In addition, such transfer cases also typically allow both high and low gear ratios.
To provide the above modes of operation, a plurality of shift collars are moved between locations wherein they drive one or two axles and either high or low gear ratios. An actuator assembly typically includes a motor driving a worm. The worm engages a worm gear, and a shaft is attached to the worm gear. An actuator pin is driven by the shaft to move the shift collars between several positions at which the various drive modes are provided.
In one example, the worm must drive the worm gear through a range of approximately 270.degree. to provide the modes of operation. A groove in the shaft moves the actuator pin between any one of several distinct axial positions. In the axial positions, the pin actuator moves shift collars between locations wherein one or two axles are driven, and also between high and low gear ratios. By selectively rotating the worm gear, one can move the pin to a desired location. The motor is preferably a reversible motor such that the actuator pin can be moved back and forth between the positions.
Problems have arisen with such transmission in the prior art. In particular, it is undesirable to allow the worm gear to rotate beyond 360.degree.. If the worm gear approaches 360.degree. of rotation, then the actuator pin or shift collar can lock the system. In the past, this has sometimes occurred and an operator is unable to change the mode of operation. This would, of course, be undesirable.
To that end, a stop has been provided on the worm gear to prevent rotation from approaching 360.degree.. In one prior art example, a stop pin extends axially from a face of the worm gear and contacts a stop surface within the worm gear housing to limit the gear to approximately 330.degree. of rotation. In this way, the stop pin prevents the gear from approaching 360.degree. of rotation.
In the prior art systems, the angular motion typically required to achieve the required movement of the actuator pin is smaller than that allowed by the stop. In the prior art systems, only 270.degree. of rotation is necessary to actuate or move the actuator pin between all axial positions, while the worm gear has a 330.degree. of range. Thus, the stop is typically not actuated, as the transmission should in theory never approach the extremes of the stop. A position feedback member is utilized to stop the worm gear at the 270.degree..
However, in practice it does happen that the worm gear occasionally reaches the end of its range of travel and the stop is required. In the past, the stop has sometimes wedged into the housing structure and lobe the worm gear. This is undesirable.